Signal generation instruments perform many functions necessary to the testing, operation and maintenance of modern electronic applications. These signal generation instruments include pulse generators, pattern generators, data generators, pseudo-random bit sequence (“PRBS”) generators, controllable jitter injection, and timing generators. The digital waveforms and data signals generated by these instruments, such as generating digital pulses, high-speed clock signals, square waves, and flexible serial or parallel bit patterns and data streams, may be utilized in many applications requiring pulse and data generation. Applications include frequency upconversion, time-domain reflectometry, emissions testing, and phase coherency, among many other applications.
A typical pulse train generated by these signal generation instruments has some important features: (a) a frequency-domain spectrum with comb spacing equal to the inverse of the pulse-repetition frequency (“PRF”) rate; (b) a frequency-domain amplitude shape defined by the sinc function (with a max-to-null bandwidth equal to the inverse of the pulse width); and (c) a constant phase difference between adjacent combs.
However, other useful reference signals are also possible. For example, the pulse train may be modulated to spread the energy such that the peak amplitude is lower in the time domain while maintaining the same comb amplitude in the frequency domain. The pulse train may also be filtered so that only a portion of the frequency spectrum is utilized.
FIGS. 1A and 1B show the time and frequency-domain descriptions 100, 120, respectively, of an ideal pulse train. In FIG. 1A, the time-domain description 100 of an ideal pulse train 102 is shown. The separation of the rising edges, Δt 104, generally known as the pulse period, is equal to the reciprocal of the PRF, frep 122, where Δt=1/frep.
In FIG. 1B, the corresponding frequency-domain description 120 of the ideal pulse train is shown. That is, the periodic pulse train is Fourier-transformed. The amplitude spectrum of the pulse train consists of many equidistant spectral points 122, which are denoted by the circles in FIG. 1B, where the amplitude values are represented by the amplitude axis 130. The unwrapped phase spectrum of the pulse train consists of many equidistant spectral points 123, which are denoted by the triangles in FIG. 1B, where the phase values are represented by the phase axis 132. The spacing 124 of the spectral points 122 and 123 is equal to the repetition rate frep of the periodic pulse train. The width of the amplitude spectrum 128 until the first null, fo 126, is determined by the pulse width, tp 106, where fo=1/tp. The spectral width 128, therefore, increases with decreasing pulse width. The unwrapped phase spectrum 123 is constant up to the frequency of the first null 126 in the amplitude spectrum 122.
Known methods of pulse generation include the use of step-recovery diodes (“SRDs”) and non-linear transmission lines (“NLTLs”). SRDs are used for pulse generation because when switched from forward bias to reverse bias, SRDs have fast recovery time, and as a result, are capable of producing pulses with sharp and fast rise times. NLTLs are also used for pulse shaping, i.e., pulse narrowing and edge sharpening. Unfortunately, both of these methods typically have poor phase responses, have varying output level with input drive level, have high PRFs or a narrow range of PRFs, and are not easily manufactured utilizing standard surface-mount technology (which results in higher manufacturing costs).
Another type of pulse generator that uses logic gates and logic delay elements is disclosed in U.S. Pat. No. 4,583,008 titled “Retriggerable Edge Detector for Edge-Actuated Internally Clocked Parts” to Grugett. This type of pulse generator is typically used to create clock signals in digital circuits. Unfortunately, as pulse widths of logic circuits decrease to tens of picoseconds, digital circuit processes require reduced voltage swings. The reduced voltage swing also reduces the available signal-to-noise ratio of the ouput pulse. Another disadvantage of the logic pulse generator is that the input signal must be a square wave or pulse train. These signals, however, are difficult to generate using microwave frequency signal generators.
Therefore, there is a need for an improved pulse generator that has lower manufacturing costs, better phase response and output characteristics, higher signal-to-noise ratio, and that allows for PRF rates that are lower or higher than conventional pulse generators. This improved pulse generator should also be usable with input signals from conventional microwave frequency signal generators.